1. Field of the Invention
The present invention relates to a stroke end checking apparatus for a machine tool of numerical control and, more particularly, to a stroke end checking apparatus for a machine tool of numerical control, including an electric discharge machine, capable of judging the ability or inability of processing within an effective stroke region prior to actual processing.
2. Description of Prior Art
FIG. 1 is a block diagram showing an example of the structure of a stroke end checking apparatus of a machine tool of numerical control, and FIG. 5 represents the position relationship of machine coordinates with positions of table stroke ends by the stroke end checking apparatus of a conventional machine tool of numerical control.
As shown in FIG. 1, reference numeral 1 stands for a central processing unit for generating an instruction to execute a control program, to analyze a processing program, and to implement a desired processing, reference numeral 2 for a storage for storing the control program in order to control a whole system of a processing unit (hereinafter referred to as ROM), reference numeral 3 for a storage for storing the processing program and so on for numerical control (hereinafter referred to as RAM), reference numeral 4 for an operation panel through which an operator can provide a control unit with a variety of operations, reference numeral 5 for a tape for numerical control on which the processing program and so on are recorded (NC tape), reference numeral 6 for a tape reader for reading the content recorded on the tape for numerical control 5, and reference numeral 7 for a display unit for displaying the processing program, a variety of numerical data, graphic screens, and so on (hereinafter referred to as CRT). Reference numeral 8 stands for a pulse distributer for distributing into driving pulses a transfer amount in each of axial directions from an operation result of the central processing unit 1, reference numeral 9 for a servo unit in the X-axis direction, reference numeral 10 for a servo unit in the Y-axis direction, reference numeral 11 for a servo motor to be drivable in the X-axis direction by the servo unit 9, and reference numeral 12 for a servo motor to be drivable in the Y-axis direction by the servo unit 10. Furthermore, reference numeral 13 stands for a stroke end detector for generating a stroke end detecting signal to a control unit (not shown), reference alphanumeral 14a for a limit switch for detecting a stroke end in the X-axis plus direction, reference alphanumeral 14b for a limit switch for detecting a stroke end in the X-axis minus direction, reference alphanumeral 15a for a limit switch for detecting a stroke end in the Y-axis plus direction, and reference alphanumeral 15b for a limit switch for detecting a stroke end in the Y-axis minus direction.
Referring again to FIG. 5, reference numeral 21 stands for a machine reference point functioning as a reference in transferring a processing table, and reference numerals 22 and 23 stand for coordinate axes of a machine coordinates system determined on the basis of the machine reference point 21, in which reference numeral 22 denotes the coordinate axis in the X-axis direction and reference numeral 23 denotes the coordinate axis in the Y-axis direction. Reference numerals 24 to 27, inclusive, denote table stroke ends, in which reference numeral 24 denotes the table stroke end in the X-axis minus direction, 25 denotes the table stroke end in the X-axis plus direction, 26 denotes the table stroke end in the Y-axis minus direction, and 27 denotes the table stroke end in the Y-axis plus direction. Reference symbol (Xst) stands for a table stroke in the X-axis direction within a transferable scope in the X-axis direction of the processing table, reference symbol (Yst) for a table stroke in the Y-axis direction within a transferable scope in the Y-axis direction of the processing table, reference symbol (Lxs) for a distance between the machine reference point 21 and the table stroke end 24 in the X-axis minus direction, and reference symbol (Lys) for a distance between the machine reference point 21 and the table stroke end 27 in the Y-axis plus direction.
The stroke end checking apparatus for the conventional machine tool of numerical control has the structure as the conventional machine tool of numerical control has the structure as have been described hereinabove, wherein a scope within which the processing table can be transferred is restricted by the stroke ends 24, 25, 26 and 27 by detecting the stroke ends restricting the scope in which the processing table can be transferred in a two-dimensional direction--more specifically, in the X-axis direction and in the Y-axis direction. Thus, the processing table cannot be transferred beyond or outside the stroke ends. The conventional various machine tools including an electric discharge machine are designed such that the stroke ends are set in order to restrict an effective transfer range of the processing table.
Operation of the conventional stroke end checking apparatus of the machine tool of numerical control will be described hereinafter.
The central processing unit 1 executes the control program stored in the ROM 2 and analyzes the processing program, thereby outputting to the pulse distributer 8 an axial transfer signal that is the output result and, as a consequence, appropriately controlling the driving of the servo motors 11 and 12 through the servo units 9 and 10, respectively. When the stroke end signals LX+, LX-, LY+ and LY- are entered into the stroke end detector 13 from the limit switches 14a, 14b, 15a and 15b during the transfer in the axial directions, the stroke end detector 13 sends the stroke end detecting signals to the control unit (not shown) of the machine tool of numerical control, thereby immediately suspending the transfer in the axial directions of the machine tool. Accordingly, these apparatuses are devised such that, when the processing locus interferes with the stroke ends during actual processing, the transfer in the axial directions is suspended, whereby processing operation cannot be continued further so that materials being then processed become useless. Thus it is desirable to implement the stroke end checking prior to actual processing.
The stroke end checking operation to be implemented prior to actual processing will be described with reference to FIG. 6.
FIG. 6 represents the relationship of the position at which a material to be processed is mounted with a processing locus when a program is checked by transferring the conventional stroke end checking apparatus of the numerical control machine tool in axial directions.
As shown in FIG. 6, reference numeral 31 stands for a material to be processed; reference numeral 32 for a level block of the processing table; reference numeral 33 for a clamp for fixing the material to be processed 31 on the level block 32; reference numeral 34 for a processing start point, an opening of the material to be processed 31, from which the processing is started; reference numerals 35 to 39, inclusive, each for a processing locus segment on the basis of the processing program for numerical control; and reference numeral 40 for a stroke end interference point that is an intersection of the processing locus segment 36 with the table stroke end 26 in the Y-axis minus direction.
Referring to FIG. 6, the stroke end checking apparatus of the numerical control machine tool of this type is operated first by fixing the material to be processed 31 on the level block 32 with the clamp 33 and determining the position of a wire nozzle of an electric discharge machine from the processing start point 34, by executing the processing program for numerical control and transferring the processing table, and then by checking whether or not each of the processing locus segments 35, 36, 37, 38 and 39 is transferred beyond each of the table stroke ends 24, 25, 26 and 27. It is to be noted here that the processing table is merely transferred in axial directions without implementing actual processing. As shown further in FIG. 6, if the processing is transferred along the processing locus segment 36 and the locus segment would interfere with the stroke end interference point 40 at the table stroke end 26 in the Y-axis minus direction, the actual processing cannot be implemented in this situation. In this case, however, the material to be processed 31 should be fixed again on the level block 32 and then the stroke end checking operation for checking the stroke end should be carried out. The actual processing should be implemented after it has been confirmed that the processing locus segments 35, 36, 37, 38 and 39 do not interfere with each of the table stroke ends 24, 25, 26 and 27.
The stroke end checking operation will be described with reference to FIG. 7 which is a flow chart showing a stroke end checking routine to be executed by the conventional stroke end checking apparatus of the numerical control machine tool.
After the material to be processed 31 has been fixed on the level block 32 with the clamp 33, then at step S31, the tape reader 6 reads the content of the numerical control tape 5. The flow proceeds to step S32 where the processing table is transferred in the axial directions in order to implement the program check in accompany with the axial transfer of the processing table by the stroke end checking apparatus. Then at step S33, it is judged whether or not a series of the processing locus segments 35, 36, 37, 38 and 39 interfere with each of the table stroke ends 24, 25, 26 and 27. If they do not interfere with any table stroke ends, on the one hand, the flow proceeds to step S34 and the actual processing is started. If they do interfere with the table stroke end, the material to be processed 31 is fixed again on the level block 32 at step S35, and the flow goes back to step S32 where the stroke end checking operation is implemented with the processing table transferred in the axial directions, in the same manner as above.
It is to be noted that this stroke end checking operation requires the processing table to be actually transferred in the axial direction so that checking a complex design on the basis of a processing program requires an increased period of time, expenses, and labor. Accordingly, attempts have been made to use the CRT 7 for checking a program on the graphic screen in order to shorten a checking time and improve work efficiency.
The program checking operation using the graphic screens will be described with reference to FIG. 8.
FIG. 8 represents the position relationship of the machine coordinates on the graphic display screen associated with the conventional stroke end checking apparatus for the numerical control machine tool with positions of stroke ends including the table stroke ends and graphic stroke ends as will be described hereinafter. As shown in FIG. 8, reference numerals 21 to 27, inclusive, and reference symbols (Xst) and (Yst) are the same as or closely similar to those used in FIGS. 5 and 6, respectively, so that duplicate description on these reference numerals and symbols is omitted herefrom for brevity of explanation.
As shown in FIG. 8, reference numerals 41 to 44, inclusive, denote stroke ends on the display screen (hereinafter referred to as graphic stroke ends), in which reference numeral 41 stands for the graphic stroke end in the X-axis minus direction, reference numeral 42 for the graphic stroke end in the X-axis plus direction, reference numeral 43 for the graphic stroke end in the Y-axis minus direction, and reference numeral 44 for the graphic stroke end in the Y-axis plus direction. Reference numeral (Xgst) stands for a graphic stroke in the X-axis direction within a transferable scope in the X-axis direction on the display screen of the processing table and reference numeral (Ygst) for a graphic stroke in the Y-axis direction within a transferable scope in the Y-axis direction on the display screen of the processing table, as well as reference symbol (Lxgs) stands for a distance between the table stroke end 24 in the X-axis minus direction and the graphic stroke end in the X-axis minus direction and reference symbol (Lygs) for a distance between the table stroke end 27 in the Y-axis plus direction and the graphic stroke end 44 in the Y-axis plus direction.
Techniques of the checking programs using the CRT 7 in the manner as have been described hereinabove are known as disclosed, for example, in Japanese Patent Publication (kokai) Nos. 297,021/1986, 297,022/1986, and 297,024/1986. These techniques require that each of the graphic stroke ends 41, 42, 43 and 44 coincides fully with each of the table stroke ends 24, 25, 26 and 27, respectively. Thus the control program should be arranged in advance so as to specify in such a manner that the operator can actually transfer the processing table so as to reach the table stroke end, read the machine coordinates value of each of the table stroke ends 24, 25, 26 and 27 after transferal, and set them to each of the graphic stroke ends 41, 42, 43 and 44, respectively, or set the graphic stroke ends 41, 42, 43 and 44 so as to be located slightly inside each of the table stroke ends 24, 25, 26 and 27, respectively. It is noted, however, that a distance (Lxs) between the machine reference point and the table stroke end in the X-axis direction and a distance (Lys) between the machine reference point and the table stroke end in the Y-axis direction vary with individual machines to a slight extent, and a table stroke (Xst) in the X-axis direction and a table stroke (Yst) in the Y-axis direction vary with individual machines to a slight extent. Accordingly, the graphic stroke (Xgst) in the X-axis direction and the graphic stroke (Ygst) in the Y-axis direction are set to become smaller by several millimeters than the table stroke (Xst) in the X-axis direction and the table stroke (Yst) in the Y-axis direction, respectively, with an error taken into account.
The conventional stroke end checking apparatus for the machine tool of numerical control is designed such that the material to be processed 31 is to be fixed on the processing table with a margin as the processing locus segments 35, 36, 37, 38 and 39 passes in the vicinity of and inside the table stroke ends 24, 25, 26 and 27.
Accordingly, when the material to be processed 31 in a comparably large size is processed to a full extent as large as the effective stroke scope, the position at which the material is mounted on the processing table should be changed often and a longer time is required for preparation work such as settings.
Although the stroke end checking operation on the graphic display screen could overcome the shortcomings presented in the stroke end checking operation by the aid of the program check accompanying the transfer in the axial directions, namely the axial transfer, an accurate stroke end checking operation on the graphic display screen cannot be implemented due to an error (in a usual case, several millimeters) between the table stroke ends 24, 25, 26 and 27 and the graphic stroke ends 41, 42, 43 and 44, respectively. Thus, when the program checking on the graphic display screen is judged to be possible, there has been the risk that the processing locus segments 35, 36, 37, 38 and 39 interfere with the table stroke ends 24, 25, 26 and 27 at the stage of actual processing. In order to ensure a prevention of such interference, it is necessary to implement the program checking operation which accompanies the transfer in axial directions.